In my project I use a 3rd party package, I used some of the static meshes as spline mesh or combined it with a closed spline:
(The monorail is a linear spline, the station ahs a closed spline added)
In 5.4 however, all these objects have no material attached
but these are all mentioned:
As it turned out, the master material of these objects had no textures assigned, but all is present. Just the assignment is missing. Luckily it’s a small package and contains just a few textures and masters (that share most textures) I could solve the issue by do the assigning myself, but I would have expected that this was done by the migration from 5.3 to 5.4